Conventionally, optical disks or optical cards are known as optical information recording media on which information can be recorded or reproduced optically. These recording media use a semiconductor laser as a light source, and a great deal of information can be recorded or reproduced by irradiating the recording media with light that is finely focused by a lens.
At present, there is much research on the above recording media to improve their recording capacities. For example, a recording medium having a multilayer structure, where recording capacity is doubled by laminating information layers for recording or reproducing information signals, has been proposed (U.S. Pat. No. 5,726,969). Under such circumstances, read-only DVD-ROM disks including two information layers have been put into practical use.
On the other hand, optical disks that can be recorded in the user's environment have been achieved using a phase changeable material, a magneto-optical recording material, a dye material, or the like. There are two systems for recording signals on such optical disks: a sector-structure system and a continuous recording system. The former is used mainly to record data information, while the latter is used to record sound information, such as CD-Rs.
In the optical disks having a sector structure, an area for managing information to be recorded and a data area on which information signals are recorded by users are separated. However, when the recording system of a sector structure is applied to a multilayer recording medium, reproduced signals are distorted because of the recorded state of adjacent layers.
FIG. 9 shows a cross section of a conventional two-layer disk and an example of reproduced signals from an information layer. In the two-layer disk shown in FIG. 9(a), a first information layer 2 and a second information layer 4 are formed on a substrate 1, and a separating layer 3 is provided between the two information layers. On top of that, a protective substrate 5 is formed.
The first information layer 2 has a sector structure including a data area 8 for recording information signals and a sector address 9 spaced at predetermined intervals along the length of the data area. The sector address 9 is used for management information for recording/reproducing information signals. Similarly, the second information layer 4 also includes a data area 12 and a sector address 13.
In FIG. 9(a), the first information layer 2 is not recorded, whereas signals are recorded on the second information layer 4.
FIG. 9(b) shows reproduced signals from the second information layer 4. In this case, since transmissivity of the first information layer 2 is unchanged, constant signals in accordance with a pattern recorded on the second information layer 4 can be reproduced.
On the other hand, in FIG. 9(c), the first information layer 2 is recorded; FIG. 9(d) shows reproduced signals in such a case. Here, the first information layer 2 has the characteristic in which its transmissivity is increased by recording information. As shown in FIG. 9(d), the reproduced signals from the second information layer 4 have a waveform whose amplitude is increased in the area corresponding to the recording area of the first information layer 2.
As described above, in an optical disk having a sector structure, signals are recorded only on the data area, not on the sector address. Therefore, when information signals are reproduced, the amplitude of reproduced signals and the signal level fluctuate significantly depending on the recorded state of the opposite layer. In particular, when the reproduced signals from the second information layer are demodulated, reproduction errors are caused in the area corresponding to the boundary between the address portion and the data area of the first information layer, so that the recorded information cannot be demodulated correctly.
Similarly, in recording, the amount of light that reaches the second information layer varies depending on the recorded state of the first information layer, so that information does not recorded correctly.